1. Field of the Invention
The present invention relates to an apparatus for forming a paper or cardboard web from a fibrous suspension, said apparatus having a double wire section consisting of a first wire loop formed by a first wire, a second wire loop formed by a second wire and at least one headbox, from where the fibrous suspension can be fed into the space between the wire loops, said first wire loop being provided with a dewatering box placed above the wire and arranged to drain water from the fibrous suspension between the wire loops via the bottom of the dewatering box and through the first wire, said apparatus having two successive dewatering zones provided in the area of the dewatering box, where the direction of web movement in the area of the first dewatering zone is away from the dewatering box and the runs of each wire loop are substantially straight while in the area of the second dewatering zone the direction of web movement is towards the dewatering box and water drainage from the fibrous suspension is arranged to take place in a direction substantially towards the dewatering box.
2. Description of Background Art
Patent specification FI 98540 discloses a hybrid former for forming a paper or cardboard web from a fibrous suspension. In a hybrid former, a web is first formed on a single wire, whereafter the web thus partially formed is passed on to a dewatering zone formed between the two wires for final stabilization of fiber positions relative to each other. According to FIG. 1 of the aforesaid specification, fibrous suspension is supplied from the headbox to the fourdrinier wire part of the second wire loop, where water is drained from the pulpstock by means of dewaterers. The fibrous layer, i.e. the web formed on the second wire loop then moves on to the twin wire section, which consists of the area between the first wire loop and the second wire loop. At the beginning of the twin wire section, the top wire loop and the bottom wire loop form a gap where the top wire loop is brought close to the bottom wire loop at a small angle. Placed inside the wire loop is a dewatering box, which drains water from the web through the first wire by causing it to flow towards the dewatering box.
In the solution according to patent specification FI 98540, the dewatering box is divided into two dewatering zones, i.e. a first straight zone and, next to it, a second curved zone. In the curved zone, the pulp web is caused to assume a curvature determined by the radius of curvature. The compression between the wires is dependent on the radius of curvature and also on the width of the wire.
With the solution in question, a relatively uniform dewatering pressure is achieved. However, the pressure is not always sufficient to drain the water especially in the case of thick sorts of paper and cardboard.